1. Field of the Invention
The present invention generally relates to an imaging technique and, more particularly, to an automatic focus system of an imaging apparatus such as a digital camera.
2. Description of the Related Art
Generally, an electronic imaging apparatus such as a digital still camera is provided with an automatic focus (AF) system, which automatically performs a focusing operation. As a method of automatic focusing, Japanese Patent Publication No. 39-5265 discloses a control method, which is referred to as a mountain-climbing AF control and used widely. In the mountain-climbing AF control, an integral value of high-frequency components or differences in intensity between adjacent pixels is acquired from an image signal obtained on an individual field or frame basis so as to set the thus-acquired integral value to an AF evaluation value which indicates a degree of focus. Since edge portions of a photographing object are sharp in a focused state, the AF evaluation value is large. On the other hand, the AF evaluation value is small in an unfocused state. When an AF control is being performed, a focal lens, which constitutes a part of an imaging optical system, is moved so as to sequentially acquire the AF evaluation value, and, then, the lens if stopped when the AF evaluation value is at a maximum where it is regarded as a focal point.
Since an accurate focus is required generally for an imaging apparatus such as a digital still camera which takes a still picture as compared with an imaging apparatus such as a video camera which takes a motion picture, an AF operation is performed each time of taking a picture or a focusing operation is repeatedly performed always in a recording mode.
However, if the mountain-climbing control AF is applied in each shoot, an amount of movement of the focus lens is large in a distance measurement range in a macro mode or a telephoto side of a zoom camera, which results in a problem in that a time lag, that is, a shutter release time lag, is generated from a time when the operator performs a shoot start request until the shoot is actually performed.
Additionally, in a digital still camera equipped with a zoom lens of which zoom magnification can be changed by changing a focal distance, it is possible to select one of two kinds of picture range, one is a macro mode which uses, as a picture range, a distance range closer than a predetermined distance and the other is a normal picture mode which uses, as a picture range, a distance range farther than the above-mentioned predetermined distance. However, in the macro mode where a distance between a photographing object and the focal lens is extremely small and an amount of movement of the focal lens in a total distance measurement range is large, and, thus, the above-mentioned shutter release time lag is generated, which results in a problem that the shutter is not released at the right moment.
Furthermore, since the movement of the focal lens by a motor is always performed as an AF scan operation on an entire object area each time of taking a picture, a large amount of electric power is consumed, which results in decrease in a service life of a battery cell, which is normally used as a power source. Additionally, if, for example, a picture of an object is taken with wire-netting in between, the AF evaluation value has a peak at the wire-netting when an entire object area is subjected to the AF scan operation, which results in an erroneous operation such that a photographing object is faded. As measures for solving such a problem, Japanese Laid-Open Patent Application No. 54-113334 suggests a technique to limit a range to perform a focal determination. Additionally, Japanese Laid-Open Patent Applications No. 2003-230039, No. 2003-262786 and No. 9-211298 disclose techniques to limit the AF scan range, that is, a range of movement of a focal lens by a motor, to a specific range.
Specifically, Japanese Laid-Open Patent Application No. 54-113334 discloses a technique to perform a focal position detection by selecting a desired distance range from among all distance ranges, but only data in the desired range from among all scan area is used, which may be effective in preventing an erroneous determination but there is little contribution to a reduction in a time period spent on the AF operation and a reduction in an electric power consumption. Japanese Laid-Open Patent Application No. 2003-230039 discloses a technique to perform an AF scan while limiting a focus range when a release operation is performed while operating a specific button. The technique of this patent document is to limit the AF scan range to a predetermined range. Japanese Laid-Open Patent Application No. 2003-262786 discloses a technique to divide a focus range and to perform an AF scan on a designated one of the divided ranges. The technique of this patent document divides a focus range, and performs an AF scan on one of the divided focus ranges so as to perform an AF scan on the other divided focus range only when the focusing operation to said one of the divided focus range cannot be completed. Japanese Laid-Open Application No. 9-211298 disclose a technique to select and set whether to divide a range of movement of a focal lens into a motion permitted area and a motion prohibited area so as to prevent the focal lens to move into the motion prohibited area.